Entrecôte à la briarde (broiled steak with mustard) - Untested

From Monet's Kitchen by Claire Joyes, pg 

Serves 1

1 fillet steak (8oz)

4 Tbsp grainy mustard

2 Tbsps butter

1. Spread both sides of the steak with mustard, and leave for 2 hours.
2. Melt the butter in a skillet and brown the steaks in the butter until the desired doneness.